Gas turbines have combustion chambers wherein a fuel can be combusted to generate a hot gas flow to be expanded in one or more expansion stages of a turbine.
Each expansion stage can include a stator airfoil row and a rotor airfoil row. During operation, the hot gas generated in the combustion chamber passes through the stator airfoil row to be accelerated and turned, and afterwards it passes through the rotor airfoil row to deliver mechanical power to the rotor.
In a gas turbine assembly, between the inner and outer wall of the combustion chamber and the inner and outer wall of the stator airfoil row, gaps can be provided. Cooling air for cooling the combustion chamber and the stator airfoil row inner and outer walls can be ejected through these gaps into the hot gases path.
In addition, also between the stator and the rotor airfoil row inner and outer walls a gap can be provided. Cooling air can be fed through these gaps also.
As the stator airfoils extend in the paths of the hot gas, they can constitute a blockage for the hot gas flow.
Thus, stator airfoils can generate regions of high static pressure in the stagnation regions upstream of their leading edges and regions of lower static pressure in the regions in-between.
The result can be a non-uniform circumferential static pressure distribution upstream of the stator airfoil row (called bow-wave) which varies in a roughly sinusoidal manner.
This pressure distribution can cause hot gas to enter into the gaps. This should be avoided because it can cause overheating of structural parts adjacent to the gaps.
This problem has been addressed by supplying additional air (purge air) fed through the gaps at high pressure (i.e. pressure greater than the sinusoidal pressure peaks).
As a consequence, the total amount of cold air (cooling air+purge air) fed through the gaps can be much greater than that necessary for cooling of the parts making up the hot gas flow channel.
Such an excessive cold air can be undesirable, because it causes the overall power and efficiency of the gas turbine to be reduced.
In order to reduce the amount of purge air fed, U.S. Pat. No. 5,466,123 discloses a gas turbine having a stator and a rotor with gaps between their inner and outer walls.
The inner stator wall has an upstream zone (the zone upstream of the stator airfoils) that is axisymmetric, and a downstream zone (the zone in the guide vane flow channels defined by two adjacent stator airfoils) that is non-axisym metric.
This configuration of the inner stator wall can let the non-uniformities (i.e. the peaks) of the hot gases pressure in a zone downstream of the stator airfoils be counteracted, but it has no influence on the hot gases pressure upstream of the stator airfoils.
WO2009/019282 discloses a gas turbine having a combustion chamber followed by a stator (and a rotor) airfoil row. Between the inner and/or outer wall of the combustion chamber and stator airfoil row a gap can be provided through which cold air can be fed. The borders of the gaps of the stator and/or combustion chamber inner and/or outer walls have radial steps that cooperate to influence the pressure distribution in the gaps.